Existing plasma cleaning devices depend primarily on the production of a plasma in a gas environment surrounding the contaminated surface. The contaminant is then removed as the active plasma species collide with the surface. At low gas pressures (less than about 10.sup.-5 torr), either the gas density is too low for a plasma discharge to be established, or the plasma density is too low to produce significant contaminant cleaning. Consequently, existing plasma cleaning devices are not operable at high vacuum conditions, since these devices must produce the plasma in the high vacuum environment. In the context of this application, the terms "high vacuum environment" and "low pressure environment" are interchangeable.
A solution to this problem is to generate the plasma in a confined higher pressure region and allow it to flow into the high vacuum area. Prior attempts to implement this solution utilized a flow restriction to separate the high and low pressure regions. These attempts failed because the active plasma species were destroyed during passage through the flow restriction. The present invention implements a solution to this problem by generating the plasma at the point immediately before the confined gas enters the high vacuum region.
U.S. Pat. No. 3,264,508 issued to Lai et al shows a plasma torch used for cutting, welding or for flame spraying tasks involving fine or delicate work which generates a hot plasma in a manner similar to the present invention. It is significant to note that all these operations are performed in regions of normal atmospheric pressures and not in a high vacuum environment. The torch contains a source of highly ionizable gas which is passed through a tube surrounded by a wire coil connected to a radio frequency generator. When the power is applied to this r.f. generator, the gas within the region of the coil ionizes and is then passed through a constrictive opening before leaving the tube.
Other representative patents relating to plasma generators are U.S. Pat. No. 3,139,509 issued to Browning; U.S. Pat. No. 3,192,427 issued to Sugawara et al; and U.S. Pat. No. 3,903,891 issued to Brayshaw.